The present invention relates to process devices. More particularly, the present invention relates to process devices which coupled to process fluid through process piping.
Various types of process devices are used to measure process variables and couple to process fluid through process piping. For example, fluid flow meters are used in industrial process control environments to measure fluid flow and provide outputs related to flow indicators and process controllers. Inferential flow meters measure fluid flow in a pipe by measuring a pressure drop near a discontinuity within the pipe. The discontinuity (primary element) can be an orifice, a nozzle, a venturi, a pitot tube, a vortex shedding bar, a target or even a simple bend in the pipe. Flow around the discontinuity causes both a pressure drop and increased turbulence. The pressure drop is sensed by a pressure transmitter (secondary element) placed outside the pipe and connected by impulse lines or impulse passageways to the fluid in the pipe. These connections are also referred to as impulse piping. Reliability depends on maintaining a correct calibration. Impulse lines can become plugged over time, which also adversely affects calibration.
Disassembly and inspection of the impulse lines is one method used to identify and correct plugging of lines. Another known method for detecting plugging is to periodically add a “check pulse” to the measurement signal from a pressure transmitter. This check pulse causes a control system connected to the transmitter to disturb the flow. If the pressure transmitter fails to accurately sense the flow disturbance, an alarm signal is generated indicating line plugging. Another known method for detecting plugging is sensing of both static and differential pressures. If there is inadequate correlation between oscillations in the static and differential pressures, then an alarm signal is generated indicating line plugging. Still another known method for detecting line plugging is to sense static pressures and pass them through high pass and low pass filters. Noise signals obtained from the filters are compared to a threshold, and if variance in the noise is less than the threshold, an alarm signal can be triggered which indicates that the line is blocked.
These known methods rely on providing static pressure sensors, disassembly of the flow meter or use of an external control system for diagnostics. The methods increase complexity and reduce reliability. There is thus a need for improved diagnostic technology that can provide more predictive, less reactive maintenance to reduce cost or improve reliability.